Split out Members, Persistents and SelfKeepAlive in separate headers.

third_party/WebKit/Source/platform/heap/SelfKeepAlive.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SelfKeepAlive_h
+#define SelfKeepAlive_h
+
+#include "platform/heap/Persistent.h"
+#include "wtf/Allocator.h"
+#include "wtf/Assertions.h"
+
+namespace blink {
+
+// SelfKeepAlive<Object> is the idiom to use for objects that have to keep
+// themselves temporarily alive and cannot rely on there being some
+// external reference in that interval:
+//
+//  class Opener {
+//  public:
+//     ...
+//     void open()
+//     {
+//         // Retain a self-reference while in an open()ed state:
+//         m_keepAlive = this;
+//         ....
+//     }
+//
+//     void close()
+//     {
+//         // Clear self-reference that ensured we were kept alive while opened.
+//         m_keepAlive.clear();
+//         ....
+//     }
+//
+//  private:
+//     ...
+//     SelfKeepAlive m_keepAlive;
+//  };
+//
+// The responsibility to call clear() in a timely fashion resides with the implementation
+// of the object.
+//
+//
+template<typename Self>
+class SelfKeepAlive final {
+    DISALLOW_NEW();
+public:
+    SelfKeepAlive()
+    {
+    }
+
+    explicit SelfKeepAlive(Self* self)
+    {
+        assign(self);
+    }
+
+    SelfKeepAlive& operator=(Self* self)
+    {
+        assign(self);
+        return *this;
+    }
+
+    void clear()
+    {
+        m_keepAlive.clear();
+    }
+
+    explicit operator bool() const { return m_keepAlive; }
+
+private:
+    void assign(Self* self)
+    {
+        ASSERT(!m_keepAlive || m_keepAlive.get() == self);
+        m_keepAlive = self;
+    }
+
+    GC_PLUGIN_IGNORE("420515")
+    Persistent<Self> m_keepAlive;
+};
+
+} // namespace blink
+
+#endif // SelfKeepAlive_h